Quantitative analysis of metal alloys in the field is an essential component to such commercial applications as the sorting of recyclable scrap metal, on-site sample analysis in mining facilities, non-destructive testing in specialty metal manufacturing, and positive material inspection of alloys. X-Ray Fluorescence (xe2x80x9cXRFxe2x80x9d) is the standard technique used to measure the composition of the major and minor elementary components with atomic number greater than about 20.
FIG. 1 is a prior art system that is used for measuring the composition of metal alloys and precious metals. As shown in FIG. 1, kilovolt photons 14 from a radioactive source 24, impinge on a target 16 whose elements are to be analyzed. In front of the radioactive source is a window 17, which is typically made out of stainless steel. The fluoresced x-rays 18 are detected in an energy-dispersing detector 20 connected to electronics 28. The detector 20 is shielded from the radiation of source 24 and from any ambient radiation by a shield 22. The incident photons 14 interact with the target 16 to produce the principal types of fluorescent radiation 18 including Compton scattering, Rayleigh scattering and photoelectric emission. Compton scattering produces a scattered x-ray with a lower energy than the incident x-ray; Rayleigh scattering produces an unchanged photon energy; and photoelectric emission, which occurs when an x-ray is absorbed by an element and x-rays characteristic of the element are emitted when the atom deexcites. The energy distribution of the fluorescent radiation is the sum of the characteristic x-rays from the target elements, the scattered radiation, and background radiations unconnected with the presence of the target. The energies of the gamma rays and x-rays emitted in the decays of 241Am, 55Fe and 109Cd are given in Table I.
In order to analyze alloys and precious metals, XRF instruments must have high efficiency for exciting and detecting x-rays whose energies range from a few keV to approximately 35 keV. To attain such sensitivity for alloy analysis, the XRF instruments now deployed in the field, including those made by Niton Corporation, use several x-ray sources, each with an energy spectrum most sensitive to specific regions of the periodic table.
In the prior art XRF analyzers, the multiple radioactive sources are used in sequence and are changed by a changing module 32 so that each x-ray source is sequentially exposed to the material being analyzed. The three standard x-ray sources are 241Am, 109Cd and 55Fe, though sometimes 253Gd or 239Pu are substituted for 241Am The 59.5 keV gamma rays of 241Am makes that source sensitive to elements in the tin region (Z=50), and efficiently covers the range of elements from rhodium (Z=45) to the rare-earth thulium (Z=69). A 109Cd source is a strong emitter of 22.2 keV x-rays that are efficient for exciting the K x-ray spectra of elements from chromium (Z=25) to ruthenium (Z=44) as well as the L x-ray spectra of heavier elements from tungsten (Z=74) through uranium (Z=92); the 88 keV gamma ray is too weak for quick-time measurements. The 5.9 keV x-ray of 55Fe is effective for exciting the elements titanium (Z=22), and vanadium (Z=23). The relative sensitivities of the three sources for measuring elements are given in Table 2.
All commercially available alloy analyzers use 109Cd sources as the primary source with 55Fe used to increase the sensitivity to the lightest elements and 241Am to analyze the elements in the tin region.
Multi-source instruments have several drawbacks. One drawback is cost. The individual radioactive sources are expensive and adding additional radioactive sources increases the cost proportionally. Second, when testing is performed on a material, the radioactive sources are used sequentially to minimize interference. Using the sources sequentially is very time consuming. Third, in order to use the source in a sequential manner, each source requires a source-changing mechanism, increasing the cost, size and complexity of the analyzer. Fourth, the multi-source system has issues of normalization and mechanical reproducibility.
Although a single source instrument would provide distinct advantages and overcome the inherent problems described above, certain prohibitions have caused the reliance on multi-source instruments. First, there is no known single radioactive source that provides a usable energy spectrum when used with the prior art XRF analysis methods. For example, an 241Am source has a spectrum with strong monoenergetic photons emitted in the range from 13.9 keV to 26.4 keV and previous analytic methods were unable to quantify this region due to the interfering Rayleigh and Compton scattering intensities that depended on the material being analysed.
In a first embodiment of the invention there is provided a device for photon fluorescence. The device includes a single radioactive source, such as 241Am. Both the emitted x-rays and gamma rays are used to determine the composition of a test material, such as a metal alloy or a precious metal that contains trace elements. An energy detector is used for receiving the fluoresced x-rays and gamma rays from the test material. The energy detector passes a signal to electronics for processing. The electronics process the signal and determine the composition of the test material based in part on the fluoresced x-rays and gamma rays. The electronics compensate for interfering Rayleigh and Compton scattering peaks by first choosing a Rayleigh scattered peak in a region of the spectrum that does not interfere with any fluoresced x-ray from the metal sample. This is the reference peak for the spectrum. For metal samples, the intensity of Rayleigh scattering through 180xc2x0 is sufficiently independent of Z that the intensity of the reference peak determines the intensity of all the other Rayleigh scattered lines. Specifically, the Rayleigh scattered spectrum from a typical metal such as iron is stored in the device""s computer. The intensity of the reference line in the sample spectrum is compared to the intensity of the reference line in the stored spectrum and the ratio is applied to the stored spectrum, which is then subtracted out of the sample spectrum. In this way, one accounts for the interfering Rayleigh peaks in the measured sample spectrum. The intensity of Compton scattering in the 12 keV to 20 keV range is low enough, from metals heavier than titanium, that they can be taken into account as well from the reference spectrum. The invention is illustrated with the use of 241Am since this source is traditionally used as a source of only 59.5 keV gamma rays. The technique can be usefully employed with other sources, for example 239Pu. If that source is used with a beryllium exit window so that the L lines are used and not absorbed, then the 12.6 keV Lxcex1 line is the appropriate normalizing Rayleigh peak.
The device further includes a shield for the radioactive source. The shield isolates the detector from direct exposure to the x-rays and gamma rays of the radioactive source so that the detector mainly receives the fluoresced radiation from the test material. The shield surrounds the radioactive source except in the direction of the test material. A source backing may be selected such as Rhodium so that the radioactive material interacts with the source backing to produce photons which combine with the x-rays and gamma rays of the source to increase the fluoresced radiation of the test material.
In certain embodiments, the shape of the shield is ring-shaped and holds the radioactive material wherein the energy detector resides inside of the ring. In another embodiment, the radioactive material is in the center and the energy detector is effectively ring-shaped around the source.